A marine vessel moves in six axes, three translational (surge, sway and heave) and three rotational (roll, pitch and yaw), see FIG. 1. A dynamic positioning system for a surface vessel usually controls only the three movements in the horizontal plane, namely surge, sway and yaw, but it may need to take into account measurements on all six axes.
The fundamental components of a general dynamic positioning system are: one or more position reference systems to measure the vessel position and heading; thrusters to apply control action; and a controller to determine the required thrusts. Typically, the object of a dynamic positioning system is not to hold a vessel absolutely stationary, but to maintain its station within acceptable limits. The magnitude of the permitted position variation is dependent upon the application and on operational concerns. In many applications a loss of position beyond the acceptable limits may have a severe impact either on the safety of personnel or equipment, or on the environment.
The present invention relates to moored marine vessels, wherein the vessels are moored by one or more anchor lines. Such vessels will have external forces acting upon them. In particular, external forces acting upon a moored vessel include the net anchor pattern force and environmental forces such as wind and current. The net anchor pattern force is the net force acting on the vessel in the surge and sway axes from the, or each, anchor line mooring the vessel. Ignoring all other external forces, the forces acting on a marine vessel from the, or each, anchor line mooring the vessel result in an anchor pattern centre where the net anchor pattern force is zero. If a moored vessel is displaced from its anchor pattern centre a net anchor pattern force will act on the vessel. The magnitude of the net anchor pattern force increases with the displacement of the vessel from the anchor pattern centre. The net anchor pattern force acts on the vessel in a direction generally towards the anchor pattern centre.
Vessels can be moored in various ways. Generally, vessels are either spread moored or turret moored.
A typical spread mooring system utilizes a set of anchor lines, normally arranged in a symmetrical pattern, attached somewhere to the vessel. This style of mooring maintains the vessel on location with a substantially fixed heading. That is, a spread moored vessel cannot rotate in the horizontal plane about its yaw axis. As a result, the connections between a spread moored vessel and the anchor lines can be relatively simple.
A turret mooring system also utilizes a set of anchor lines, normally arranged in a symmetrical pattern, attached somewhere to the vessel. However, in a turret mooring system the vessel is free to weathervane. That is, a turret moored vessel can rotate in the horizontal plane about its yaw axis into a direction where environmental loading due to wind, waves and current is minimised. This is achieved by connecting the anchor lines to a turret mounted on the vessel which, via bearings, allows the vessel to rotate independently of the anchor lines. A turret of a vessel can be mounted on the vessel either internally or externally. An external turret may be mounted, with appropriate reinforcements, on the bow or stern of the ship. If the turret is mounted internally within the vessel it may be mounted within the hull of the vessel, in a moon pool. In this case a chain table of the mooring system, which connects the anchor lines to the turret, can be either above or below the waterline.
Without thruster assistance a moored vessel will move towards or settle in a position where the net anchor pattern force acting on the vessel is equal and opposite to the net environmental force acting on the vessel. Depending upon the environmental conditions, this position may be some distance from the anchor pattern centre. If the net environmental force is large, for example during heavy weather, the vessel may move towards or settle in a position that is a significant distance from the anchor pattern centre. This can result in severe stress on some or all of the anchor lines. Furthermore, if and when the environmental conditions change and the net environmental force varies as a result, the position towards which a vessel is moving, or in which it is settled, will change. The change can be significant and may result in significant position excursions, especially during heavy weather. Such excursions are highly undesirable due to the possibility of damage to risers, entanglement with anchor lines, or excessive tension being applied to one or more anchor line. In order to minimise excursions, thruster assisted mooring systems are often used to hold a moored vessel's position substantially at a target position.
Thruster assisted mooring systems are specialised dynamic positioning systems that are used when a vessel is moored. A thruster assisted mooring system may control a vessel's movement in one or more of its surge, sway and yaw axes. Movement about or along the vessel's remaining axes is not controlled. A typical thruster assisted mooring system will attempt to maintain a vessel at or near a target position. The target position may be at a distance from the net anchor pattern centre. Thruster assisted mooring has two main purposes: i) to maintain the position of a moored vessel and thereby prevent excessive strain in the, or each, anchor line and other equipment attached to the vessel, e.g. risers which are also connected to the seabed; and ii) to reduce natural oscillations of the vessel caused by the resonance of the anchor pattern and environmental forces from waves, wind or current. The need to maintain position in the region of a riser is described, for example, in GB 1486158. The requirement to reduce natural oscillations of a vessel is stated in the standard DNV-OS-E301, Det Norska Veritas, Norway, October 2008.
The controller of a thruster assisted mooring system may take many forms. Model-based controllers have been utilised for thruster assisted mooring. However, the use of model-based controllers for thruster assisted mooring systems requires either measurement of anchor line forces, or complex models of anchor line catenaries in order to predict the anchor line forces (Jenman, C. “Mixing dynamic positioning and mooring” Marine Technology Society Dynamic Positioning Conference 2005, 15-16 November 2005, Houston, Tex., USA). Three-term controllers, also known as proportional-integral-differential (PID) controllers, are widely used in many applications and have been used for thruster assisted mooring. The advantage of using PID controllers for thruster assisted mooring is that they do not require anchor force measurements or a model of the anchor lines. It is also relatively straightforward to tune a PID controller for a particular system. Tuning of a PID controller for a thruster assisted mooring system may include zeroing the proportional term in the control calculation for one or more axes, this produces an integral-differential (ID) controller
The controller of many known thruster assisted mooring systems acts to maintain a vessel at a target position. As set out above, this is not always necessary. In order to avoid or reduce unnecessary use of thrust, and thereby minimise energy usage, it is often preferable that a moored vessel is not held precisely at a target position but is instead maintained in an area around a target position within which the positional variations of the vessel are within acceptable limits. For example, GB1486158 discloses a thruster assisted mooring system wherein the vessel is maintained in a region surrounding a target position, rather than precisely at the target position.
For turret-moored systems, a number of specific control algorithms have been proposed for minimising energy usage of thruster-assisted mooring systems. Aamo and Fossen (Aamo, O. M. and Fossen, T. I., “Controlling line tension in thruster assisted mooring systems”, Proc. of the IEEE Int. Conf on Control Applications (CCA '99), Hawaii, Aug. 22-26, 1999) alter the line tensions using the windlasses to obtain optimum thrust usage, while Berntsen et al (Berntsen, P. I. B., Aamo, O. M. and Leira, B. J., “Thruster assisted position mooring based on structural reliability”, Int. Journal of Control, 81 (9), pp. 1408-1416, 2008) utilise a model of structural reliability to calculate allowable reductions in thrust. discloses the use of an ID controller on the surge axis with a target range within which the thrust demand is reduced, and outside which the thrust demand is increased. The control algorithm.
The present invention relates to spread moored vessels. Whilst, as set out above, there are many proposed control algorithms for thruster assisted mooring systems for turret-moored vessels, there are very few proposed control algorithms for thruster assisted mooring systems for spread-moored vessels. One reason for this is that the heading of turret moored vessel can be easily controlled whilst the heading of a spread moored vessel is substantially constant.
In light of the above, there is a need for a method of controlling the position of a spread moored vessel in which unnecessary use of thrust and the resulting energy usage is minimised whilst the position of the vessel is also maintained within acceptable limits without the need for anchor force measurements or models of the anchor lines. Preferably, when the forces acting on the spread moored vessel are large enough to move the vessel outside of the acceptable limits use of the method should result in the vessel being held within a position within the acceptable limits that minimises the thrust that needs to be applied to the vessel